1. Field of Invention
The invention relates generally to a subsea wellhead assembly. More specifically, the invention relates to a casing hanger seal having an energizing ring that is locked in a set position by a lockdown member.
2. Description of Prior Art
Seals are topically inserted between subsea inner and outer wellhead tubular members to contain internal well pressure. The inker wellhead member is generally a banger for supporting either easing or tubing that extends mm the well. Outer wellhead members are wellhead housings or they can be a easing hanger when the inner member is a tubing hanger. A variety of seals located between the inner and outer wellhead members is known. Examples of known seals are elastomeric metal and combinations thereof. The seals may be set by a running tool, or they may be set in response to the weight of the string of easing or tubing. One type of metal-to-metal seal has seal body with inner and outer walls separated by a cylindrical slot, forming a “U” shape. An energizing ring pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The energizing ring is typically a solid wedge-shaped member. During setting, the deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
The seal surfaces on the inner and outer wellhead members may have wickers. Wickers comprise a set of annular parallel grooves formed m the seal surface. Typically, the wickers have saw tooth shapes, defining valleys and crests. The setting tool forces the seal surfaces of the seal assembly against the wickers, causing the crests to embed into the seal surface.
Under certain circumstances high pressure cycles and temperature cycles can cause the energizing ring to move slightly upward. The upward movement of the energizing ring would cause a loss in the seal pressure rating. Proposals to prevent the upward loosening movement of the energizing ring include a variety of engagements between the energizing ring and the seal member.
It is also known to set a lockdown hanger or member in a wellhead housing bore above a casing hanger to prevent upward movement of the casing hanger. In one type, a split ring, when actuated, mates with annular retaining grooves formed in the wellhead housing bore. A wedge ring moves the split ring into the retaining ring grooves. The lockdown hanger running tool causes an actuating ring to expand a retainer ring into frictional engagement with the wellhead housing and the lockdown hanger to hold the wedge ring in the lower position. The lower end of this type of lockdown hanger engages only the casing hanger, not the seal assembly.